The catalytic cracking process is one of the major refining operations which is currently employed in the conversion of petroleum to desirable fuels such as gasoline and diesel fuel. The fluidized catalytic cracking process is an example of this type of process wherein a high molecular weight hydrocarbon feedstock is converted to lower molecular weight products through contact with hot, finely-divided solid catalyst particles in a fluidized or dispersed state. Suitable hydrocarbon feedstocks typically boil within the range of from about 205.degree. C. to about 650.degree. C., and they are usually contacted with the catalyst at temperatures in the range from about 450.degree. C. to about 650.degree. C. Suitable feedstocks include various mineral oil fractions such as light gas oils, heavy gas oils, wide-cut gas oils, vacuum gas oils, kerosenes, decanted oils, residual fractions, reduced crude oils and cycle oils which are derived from any of these as well as fractions derived from shale oils, tar sands processing, and coal liquefaction. Products from the process are typically based on boiling point and include light naphtha (boiling between about 10.degree. C. and about 221.degree. C.), kerosene (boiling between about 180.degree. C. and about 300.degree. C.), light cycle oil (boiling between about 221.degree. C. and about 345.degree. C.), and heavy cycle oil (boiling at temperatures higher than about 345.degree. C.).
Not only does the catalytic cracking process provide a significant part of the gasoline pool in the United States, it also provides a large proportion of the sulfur that appears in this pool. The sulfur in the liquid products from this process is in the form of organic sulfur compounds and is an undesirable impurity which is converted to sulfur oxides when these products are utilized as a fuel. These sulfur oxides are objectionable air pollutants. In addition, they can deactivate many of the catalysts that have been developed for the catalytic converters which are used on automobiles to catalyze the conversion of harmful emissions in the engine exhaust to gases which are less objectionable. Accordingly, it is desirable to reduce the sulfur content of catalytic cracking products to the lowest possible levels.
The sulfur-containing impurities of straight run gasolines, which are prepared by simple distillation of crude oil, are usually very different from those in cracked gasolines. The former contain mostly mercaptans and sulfides, whereas the latter are rich in thiophene derivatives.
Low sulfur products are conventionally obtained from the catalytic cracking process by hydrotreating either the feedstock to the process or the products from the process. The hydrotreating process involves treatment with elemental hydrogen in the presence of a catalyst and results in the conversion of the sulfur in the sulfur-containing organic impurities to hydrogen sulfide which can be separated and converted to elemental sulfur. Unfortunately, this type of processing is typically quite expensive because it requires a source of hydrogen, high pressure process equipment, expensive hydrotreating catalysts, and a sulfur recovery plant for conversion of the resulting hydrogen sulfide to elemental sulfur. In addition, the hydrotreating process can result in an undesired destruction of olefins in the feedstock by conversion to saturated hydrocarbons through hydrogenation. This destruction of olefins by hydrogenation is undesirable because it results in the consumption of expensive hydrogen, and the olefins are valuable as high octane components of gasoline. As an example, naphtha of a gasoline boiling range from a catalytic cracking process has a relatively high octane number as a result of the presence of a large olefin content. Hydrotreating such a material causes a reduction in the olefin content in addition to the desired desulfurization, and octane number decreases as the degree of desulfurization increases.
During the early years of the refining industry, sulfuric acid treatment was an important process that was used to remove sulfur, precipitate asphaltic material, and improve stability, color and odor of a wide variety of refinery stocks. At page 3-119 of the Petroleum Processing Handbook, W. F. Bland and R. L. Davidson, Ed., McGraw-Hill Book Company, 1967, it is reported that low temperatures (-4.degree. to 10.degree. C.) are used in this process with strong acid, but that higher temperatures (21.degree. to 54.degree. C.) may be practical if material is to be rerun. It is disclosed in the Oil and Gas Journal, Nov. 10, 1938, at page 45 that sulfuric acid treatment of naphtha is effective in removing organic sulfur-containing impurities such as isoamyl mercaptan, dimethyl sulfate, methyl-p-toluene sulfonate, carbon disulfide, n-butyl sulfide, n-propyl disulfide, thiophene, diphenyl sulfoxide, and n-butyl sulfone. The chemistry involved in sulfuric acid treatment of gasoline is extensively discussed by G. E. Mapstone in a review article in the Petroleum Refiner, Vol. 29, No. 11 (November, 1950) at pp. 142-150. Mapstone reports at page 145 that thiophenes may be alkylated by olefins in the presence of sulfuric acid. He further states that this same reaction appears to have a significant effect in the desulfurization of cracked shale gasoline by treatment with sulfuric acid in that a large proportion of the sulfur reduction obtained occurs on the redistillation of the acid treated gasoline, with the rerun bottoms containing several percent of sulfur.
U.S. Pat. No. 2,448,211 (Caesar et al.) discloses that thiophene and its derivatives can be alkylated by reaction with olefinic hydrocarbons at a temperature between about 140.degree. and about 400.degree. C. in the presence of a catalyst such as an activated natural clay or a synthetic adsorbent composite of silica and at least one amphoteric metal oxide. Suitable activated natural clay catalysts include clay catalysts on which zinc chloride or phosphoric acid have been precipitated. Suitable silica-amphoteric metal oxide catalysts include combinations of silica with materials such as alumina, zirconia, ceria, and thoria. U.S. Pat. No. 2,469,823 (Hansford et al.) teaches that boron trifluoride can be used to catalyze the alkylation of thiophene and alkyl thiophenes with alkylating agents such as olefinic hydrocarbons, alkyl halides, alcohols, and mercaptans. In addition, U.S. Pat. No. 2,921,081 (Zimmerschied et al.) discloses that acidic solid catalysts can be prepared by combining a zirconium compound selected from the group consisting of zirconium dioxide and the halides of zirconium with an acid selected from the group consisting of orthophosphoric acid, pyrophosphoric acid, and triphosphoric acid. It is further disclosed that thiophene can be alkylated with propylene at a temperature of 227.degree. C. in the presence of such a catalyst.
U.S. Pat. No. 2,563,087 (Vesely) discloses that thiophene can be removed from mixtures of this material with aromatic hydrocarbons by selective alkylation of the thiophene and separation of the resulting thiophene alkylate by distillation. The selective alkylation is carried out by mixing the thiophene-contaminated aromatic hydrocarbon with an alkylating agent and contacting the mixture with an alkylation catalyst at a carefully controlled temperature in the range from about -20.degree. C. to about 85.degree. C. It is disclosed that suitable alkylating agents include olefins, mercaptans, mineral acid esters, and alkoxy compounds such as aliphatic alcohols, ethers and esters of carboxylic acids. It is also disclosed that suitable alkylation catalysts include the following: (1) The Friedel-Crafts metal halides, which are preferably used in anhydrous form; (2) a phosphoric acid, preferably pyrophosphoric acid, or a mixture with sulfuric acid in which the volume ratio of sulfuric to phosphoric acid is less than about 4:1; and (3) a mixture of a phosphoric acid, such as orthophosphoric acid or pyrophosphoric acid, with a siliceous adsorbent, such as kieselguhr or a siliceous clay, which has been calcined to a temperature of from about 400.degree. to about 500.degree. C. to form a silico-phosphoric acid combination which is commonly referred to as a solid phosphoric acid catalyst.
U.S. Pat. No. 2,943,094 (Birch et al.) is directed to a method for the removal of alkyl thiophenes from a distillate which consists predominately of aromatic hydrocarbons, and the method involves converting the alkyl thiophenes to sulfur-containing products of a different boiling point which are removed by fractional distillation. The conversion is carried out by contacting the mixture with a catalyst at a temperature in the range from 500 to 650.degree. C., wherein the catalyst is prepared by impregnating alumina with hydrofluoric acid in aqueous solution. It is disclosed that the catalyst functions to: (1) convert alkyl thiophenes to lower alkyl thiophenes and/or unsubstituted thiophene by dealkylation; (2) effect the simultaneous dealkylation and alkylation of alkyl thiophenes; and (3) convert alkyl thiophenes to aromatic hydrocarbons.
U.S. Pat. No. 2,677,648 (Lien et al.) relates to a process for the desulfurization of a high sulfur olefinic naphtha which involves treating the naphtha with hydrogen fluoride to obtain a raffinate, defluorinating the raffinate, and then contacting the defluorinated raffinate with HF-activated alumina. The treatment with hydrogen fluoride is carried out at a temperature in the range from about -51.degree. to -1.degree. C. under conditions which result in the removal of about 10 to 15% of the feedstock as a high sulfur content extract, and about 30 to 40% of the feedstock is simultaneously converted by polymerization and alkylation to materials of the gas oil boiling range. After removal of HF from the raffinate, the raffinate is contacted with an HF-activated alumina at a temperature in the range from about 316 to 482.degree. C. to depolymerize and dealkylate the gas oil boiling range components and to effect additional desulfurization.
U.S. Pat. No. 4,775,462 (Imai et al.) is directed to a method for converting the mercaptan impurities in a hydrocarbon fraction to less objectionable thioethers which are permitted to remain in the product. This process involves contacting the hydrocarbon fraction with an unsaturated hydrocarbon in the presence of an acid-type catalyst under conditions which are effective to convert the mercaptan impurities to thioethers. It is disclosed that suitable acid-type catalysts include: (1) acidic polymeric resins such as resins which contain a sulfonic acid group; (2) acidic intercalate compounds such as antimony halides in graphite, aluminum halides in graphite, and zirconium halides in graphite; (3) phosphoric acid, sulfuric acid or boric acid supported on silica, alumina, silica-aluminas or clays; (4) aluminas, silica-aluminas, natural and synthetic pillared clays, and natural and synthetic zeolites such as faujasites, mordenites, L, omega, X and Y zeolites; (5) aluminas or silica-aluminas which have been impregnated with aluminum halides or boron halides; and (6) metal sulfates such as zirconium sulfate, nickel sulfate, chromium sulfate, and cobalt sulfate.